The present invention relates to an optically readable card, an optically writable card, or an optically readable and writable card (hereinafter, these cards are generally referred to as an optically readable/writable card or, simply, an optical card). The term "optical/optically" also incorporates the concept of "magnetooptic/magnetooptically".
The optical card has a structure such that a recording medium layer is arranged on a part or all of the upper surface of a card substrate and a transparent protection layer made of a transparent plastic material is laminated on the card substrate and recording medium layer so as to cover the recording medium layer.
Predetermined data is represented and recorded as forms of positions, sizes, or the like of a number of pits formed on the recording medium layer. A laser beam is focused so as to have a diameter of about 1 .mu.m and then irradiated onto the recording medium layer. The reflected lights are received by a sensor. The data is read out using of the principle such that the intensity of reflected lights is modulated by the existence of the pits. In this manner, the purely optically readable card is constituted.
The recording principle of the magnetooptically writable/readable card is as follows. The light is irradiated onto the recording medium layer to thereby locally increase the temperature thereof and at the same time, the magnetism is applied to the recording medium layer from the outside to thereby change the direction of the magnetization of the local portion. By focusing the laser beam, a temperature of the fairly small area having a diameter of about 1 .mu.m can be raised, so that the high density recording can be performed. In general, the magnetic field necessary for the magnetic recording decreases with an increase in temperature of the recording medium; therefore, data can be recorded even by an extremely weak magnetic field.
At present, two kinds of recording and erasing methods are considered. One is a magnetic field modulation method whereby a laser beam is always irradiated onto the recording medium and the magnetic field to be applied is changed in accordance with the data to be recorded. The other is the optical modulation method whereby a DC magnetic field is always applied and a laser beam to be irradiated is allowed to flicker in accordance with the data to be recorded.
The optical reading method includes the direct optical playing-back method and the indirect optical playing-back method. The former method uses the principle such that when a linearly polarized light is directly irradiated to the recording position, the polarizing direction of the reflected lights (or transmitted light) rotates due to the magnetooptic effect. According to the latter method, after a recording pattern was transferred onto a thin magnetic film, it is read out with the light.
In such an optical card, since the recorded information is read out or written by the optical method, if the surface of the transparent protection layer is scratched, there is a fear such that it is difficult to read out the recorded information from the recording medium layer or the recorded information is erroneously read out or the like.
If the optical card is used as such a conventional magnetic card that it is always carried and frequently pulled out from and put into a card casing, the surface of the card will be soon scratched or the like. Consequently, there is a limitation when the optical card is used in a wide application field such as in the magnetic card.